Polyols and especially neopentyl glycol (NPG; 2,2-dimethyl-1,3-propanediol) are widely used as starting materials for preparation of various useful products such as lubricants, plastics, surface coatings, surfactants, and synthetic resins. Polyalcohols like NPG are typically produced by a two-step process. The first step is an Aldol condensation of an enolizable aldehyde, such as isobutyraldehyde, with formaldehyde to form a hydroxy aldehyde intermediate such as hydroxypivaldehyde (HPA). The second step is the hydrogenation of the hydroxy aldehyde over a metal containing catalyst to form the polyalcohol such as NPG as shown in Scheme 1.
Scheme 1 Preparation of Polyalcohols by Aldol Condensation and Hydrogenation

A parameter to watch in the first step of the preparation of polyols is how efficiently the reactants are converted to the hydroxy aldehyde intermediate. The formaldehyde concentration in the Aldol product is frequently considered as an indicator of the efficiency of the reaction because the levels of formaldehyde in the Aldol product can be readily measured by a number of analytical techniques.
Although a large number of catalysts have been previously published, the commercially viable Aldol condensation catalysts can be divided into two groups: (1) a strong alkaline catalyst such as sodium hydroxide or sodium carbonate and (2) a tertiary amine such as TMA or triethylamine.
Alkaline Catalyst Systems
The alkaline catalyst systems have been publically known for many years. In general, these systems are biphasic and consist of a mixture of aldehyde and an aqueous formalin solution. The alkaline catalyst is usually consumed during the process by side reactions such as the Cannizzaro reaction which forms salts of the carboxylic acids that correspond to the aldehydes. Examples of these acids are formic acid, isobutyric acid and hydroxypivalic acid. The salts of the acids need to be removed from the stream prior to distillation and hydrogenation to prevent breakdown to retro Aldol products in the distillation column and hydrogenation reactor. When an excessive amount of formaldehyde is reacted with aldehyde in the presence of a strong alkaline catalyst, large amounts of formate salts are formed as byproducts, making this process commercially unsuitable. On the other hand, when an excessive amount of aldehyde is employed, the excessive amount of aldehyde reacts i) with the product to form esters or ii) with itself to form Aldols and acetals. These byproducts require several additional steps for the purification process and ultimately result in yield loss.
Tertiary Amine Catalyst Systems
The tertiary amine catalyst systems are usually run at a molar ratio with an excessive amount of aldehyde which enables the reaction to be carried out in a homogeneous reaction mixture. In these processes, the selectivity of Aldol is increased compared to the alkaline catalyst systems. The use of tertiary amine catalysts in the Aldol condensation is not perfect.
The tertiary amine catalysts react with organic acids such as formic acid to form salts. Formic acid exists in commercial formaldehyde raw material.
Formaldehyde also reacts with isobutyraldehyde and HPA to form isobutyric acid and hydroxypivalic acid. These acids form salts with the tertiary amine catalyst.
The amine salts cannot be separated from the hydroxy aldehyde by distillation. These amine salts are carried on into the hydrogenation reactor, contacting the metal catalyst therein. The amine salts can deactivate the metal catalyst in the hydrogenation reaction. Further, the amine salts can promote the decomposition of the Aldol condensation product during the distillation of product at high temperatures. Thus, overall yields are dramatically decreased. The amine salts can also cause undesired color and/or odors in the downstream products.